There's a difference between being safe and being unrealistic. Sakurai merely said not to get expectations too high. As in, don't expect us to get as many characters for this game as last.
What has been said that makes it reasonable to conclude we're getting half of the amount we got in Melee, the most rushed game in the history of Smash Bros. and the one with the least newcomers?
It's mainly the scope of the game. Bringing back everyone ever in the series is no small task, not to mention that characters aren't the only thing that has to be worked on. Stages, modes, trophies, music, there's a toooon of stuff that all requires a ton of time to get right, and there's only so much a single development team can do, especially when the game most likely only has 5 more months of dev time left before it has to go gold for it's Dec. 7th release. And for the characters themselves, sure, you can reuse some things, but interviews coming from Nintendo are stating most of this game is being done from scratch, so that means new models, new animations, textures, voice work, etc. It all piles up after a while.
I'm not saying there won't be any more newcomers, I'm just expecting the bulk of them to come from DLC.
